Scan-with-compensation is a method of radar tracking which, like the more familiar conical scan tracking, generates a tracking error signal from the modulation of a target echo which results from rotating about the antenna scanning axis an antenna pattern which is offset in angle or squinted by an amount which is typically one-half to one-quarter of the half power one-way beam width of the antenna. Unlike conical-scan tracking, the antenna feed is used to generate two pencil-beam radiation patterns which are squinted by equal but opposite angles with respect to the scan axis. This type of radiation pattern is illustrated in FIG. 1 of U.S. Pat. No. 4,028,708 issued June 7, 1977 to the present applicant.
In the aforementioned patent, it was shown how by the use of mode transducers between the TE.sub.11 mode of circular waveguide and the TE.sub.10 mode of rectangular guide, two side-by-side parallel circular guide radiators could be connected to a third circular guide, the central axis of which lies in prolongation of the line parallel to and midway between the two circular guide radiators. By one disposition of the innerconnections of the mode transducers on the three waveguides, the aforementioned patent demonstrates that whatever polarization of the TE.sub.11 mode was established in the axial guide would be preserved in the two parallel guide radiators and would not change as the entire assembly of three guides and their connections rotated about the axis of the assembly. By another innerconnection, the polarization in one of the parallel guides would be the same as that in the axial guide but the polarization in the other parallel guide would be perpendicular; by symmetry and reciprocity, it followed that if waves of the same polarization, but of course possibly differing in amplitude, phase, or modulation, were introduced into the two parallel guides, the wave in each of these guides would give rise to an independent field component in the axial guide and these latter components would be orthogonal in polarization and thereby separable. This behavior is likewise independent of the rotation of the guide assembly about its axis.
The second above described configuration is suitable for scan-with-compensation radar tracking as the focused beams of the two parallel radiators rotate together about the tracking axis with equal and opposite squint angles and the signals received by them are orthogonal and hence separable after passing from the axial guide through a choke joint to a non-rotating circular guide.
The scan-with-compensation tracker preferaby has an unmodulated, axially directed illumination field when transmitting. This can be approximated by the sum field of the parallel guide radiators when connected in the first mentioned manner so that both radiator emit fields with fixed parallel polarization. As the assemblage rotates about its axis, the sum field will be only slightly modulated in amplitude, at least at points near the boresight axis. This form of scan-with-compensation antenna feed, however, has the following problems: the necessity for switching the connections between the axial guide and the two parallel guides from the transmitting to the receiving configuration (which can be accomplished by circulators or gas-discharge switches); residual low-scan modulation; and the loss of the transmitted beam because of the high degree of squint adopted to minimize scan modulation.